It is previously known in this art how artificial implants may be anchored directly in bone tissue. In order to avoid the risk of loosening or detachment, every attempt is made to achieve a direct contact, that is an exact adaptation between the implant and the surrounding bone tissue, so-called osseointegration. Such exact adaptation may be achieved by sophisticated operational technique and by a suitable design of the implant. The osseointegration principle developed by professor Br.ang.nemark et al has successfully been used clinically for 20 years for maxillary-anchored dental bridges and is described in, for example:
P-I Br.ang.nemark et al, "Osseointegrated titanium fixtures in the treatment of edentulousness". Biomaterials, 1983. Vol 4, January; and Rickard Skalak, "Biomechanical considerations in osseointegrated prostheses". The Journal of Prosthetic Dentistry, June 1983, Volume 49, Number 6.
The principle is based on the fact that the implant is of pure titanium, at least in the interface zone between living tissue and implant. Swedish patent specification No. 79.02035-0 corresponding to U.S. Pat. No. 4,330,891 also discloses and describes the importance of the surface structure of the titanium for a powerful connection between the living bone tissue and the implant. By a special so-called microporous surface structure of the implant, the preconditions are further improved for a more or less permanent anchorage of the implant in the tissue.
To be able to assess the preconditions and potential for different implants to form a permanent anchorage with the tissue, there is a need to be able to study in greater detail and under standardized conditions the bone ingrowth of the implant and also those phases which precede the bone formation proper. Thus, it is previously known in this art how a special examination chamber may be operated into the bone tissue of a living animal, the chamber being designed such that samples of newly-formed bone tissue may be harvested from the chamber at regular intervals, and be examined. Hence, the chamber makes possible a quantification of bone ingrowth/implant incorporation under different experimental conditions without the need of sacrificing the animal. Such a test chamber, "The Harvest Chamber", is described in
T. Albrektsson, M. Jacobsson and P. Kalebo, "The Harvest Chamber--A newly developed Implant for Analysis of Bone Remodelling in situ", Biomaterials and Biomechanics 1983, pp. 283-288.
The harvest chamber is constructed from an outer portion with a central recess in which an inner, removable portion is inserted. The two portions together form a bone ingrowth through channel which is intended to be exposed when the inner portion is removed in order, by such a manner, to make possible examination of tissue which has grown into the channel.
With the assistance of the harvest chamber, the bone ingrowth may be studied under different pathological/experimental conditions and be compared with bone formation under normal conditions. For comparative tests, one examination chamber is normally inserted surgically into each extremity, for example the tibia, of the experimental animal. The one chamber acts as a test chamber and is surgically placed in that bone tissue which is exposed to, or will be exposed to, a potentially bone-stimulating/bone-retarding effect of some type, for example a locally administered test substance, while the second chamber constitutes a control reference chamber and is surgically inserted into bone tissue which is not under such influences. Evaluation of the results is affected in that bone ingrowth in the test chamber inserted in the one extremity is compared with bone ingrowth in the control reference chamber in the other extremity. Suitably, data-based microradiography is employed for quantification.
While the difference between the bone tissues, including their growth capacity, is slight under normal conditions, see for example the reference "The Harvest Chamber . . . ", FIG. 2, differences in the bone tissue between the two extremities may, in certain cases, derange the result of the evaluation.